Production of hydrogen from solar radiation at high efficiency

ABSTRACT

Method and apparatus for producing hydrogen by conversion of solar energy into thermal and electrical energy for electrolysis of steam.

The present invention relates to a method and an apparatus for theproduction of hydrogen and in particular for the production of hydrogenin an electrolysis cell using solar radiation as a source of energy forthe cell.

A present invention also relates to an apparatus for separating longerand shorter wavelength solar radiation so that the separated componentsof the solar radiation spectrum can be used as required in selectedend-use applications, such as the production of hydrogen.

The use of hydrogen as a carrier of energy, particularly in the contextas a fuel, has the following significant technical advantages over otherenergy sources.

1. Supply side considerations--hydrogen is inexhaustible, storable,transportable, and has a high energy density compared with otherchemical fuels.

2. Demand side considerations--hydrogen is non-polluting, more versatilethan electricity, more efficient then petrol, and convertible directlyto heat and electricity for both mobile and stationary applications.

By way of particular comparison, the large scale use of solar energy asan energy source has been limited for technical reasons and cost by alack of a suitable short and long term storage medium or solar energy.

However, notwithstanding the above technical advantages of hydrogen asan energy source, the cost of production of hydrogen has been too highhitherto for widespread use as a fuel.

In the case of the production of hydrogen by electrolysis of water, amajor factor in the high cost of production has been the cost ofelectricity to operate electrolysis cells.

In the specific case of solar radiation-generated electricity, the highcost of electricity is due in large part to the relatively lowefficiency of photovoltaic (or thermal) conversion of solar energy intoelectricity which means that a relatively large number of photovoltaiccells (or, in the case of thermal conversion, a large collection area)is required to generate a unit output of electricity.

An object of the present invention is to provide a solar radiation basedmethod and apparatus for producing hydrogen in an electrolysis cellwhich has a significantly higher efficiency and thus lower cost per unitenergy produced than the known technology.

Another object of the present invention is to provide an apparatus forseparating longer and shorter wavelength components of the solarradiation spectrum such that the separated components can be usedefficiently.

According to a first aspect of the present invention there is provided amethod of producing hydrogen comprising, converting solar radiation intothermal energy and electrical energy, and using the thermal energy andthe electrical energy for producing hydrogen and oxygen by electrolysisof water.

The above first aspect of the present invention is based on therealisation that when the electrolysis process is run at hightemperature (1000°C) the electrical voltage required to maintain a givenoutput of hydrogen can be reduced provided there is a complementaryincrease in thermal energy input.

The above first aspect of the present invention is based on therealisation that a significant improvement in efficiency of energyutilisation over and above a conventional electrolysis cell that isoperated solely by electrical energy generated from solar radiation by aphotovoltaic cell (or by thermal electrical generation methods) can beachieved by using the thermal energy produced in the generation ofelectrical energy, which otherwise would be regarded as a waste lowtemperature heat (with a cost of disposal), with the solar generatedelectrical energy to operate the electrolysis cell.

The above first aspect of the present invention is also based on therealisation that such waste thermal energy can only be used toadvantage, in terms of efficiency of energy utilisation, if that thermalenergy can be transferred to the electrolysis cell and produce the hightemperatures necessary to operate the electrolysis cell.

It is preferred that the method comprises separating the solar radiationinto a shorter wavelength component and a longer wavelength component,and converting the shorter wavelength component into electrical energyand converting the longer wavelength component into thermal energy.

It is preferred that the method comprises, producing hydrogen and oxygenby electrolysis of water by converting water into steam and heating thesteam to a temperature of at least 700° C., more preferably 1000° C.,and decomposing the steam into hydrogen and oxygen in an electrolysiscell.

It is preferred that the method comprises using solar radiationgenerated thermal energy for converting water into steam and/orpre-heating steam and for operating the electrolysis cell and usingsolar radiation generated electrical energy for operating theelectrolysis cell.

It is preferred particularly that the method comprises extractingthermal energy from hydrogen, oxygen, and exhaust steam produced in theelectrolysis cell and using the extracted thermal energy as part of theenergy component required for converting water into steam or forpre-heating steam for consumption in the electrolysis cell.

According to the first aspect of the present invention there is alsoprovided an apparatus for producing hydrogen by electrolysis comprising,an electrolysis cell having an inlet for steam and outlets for hydrogen,oxygen, and excess steam, a means for separately converting solarradiation into thermal energy and into electrical energy arranged inseries or in parallel relationship for providing the energy required forconverting water into steam and/or heating steam for operating theelectrolysis cell to decompose the steam into hydrogen and oxygen athigh temperatures of at least 700° C., more preferably at least 1000° C.

It is preferred that the electrolysis cell be at least partially formedfrom materials that allow oxygen to be separated from hydrogen in and/oradjacent to the electrolysis cell.

It is preferred that the apparatus further comprises, a means forconcentrating solar radiation on the thermal energy conversion means andon the electrical energy conversion means in the appropriate proportionsand wavelengths.

In one embodiment, it is preferred that the electrical energy conversionmeans and the thermal energy conversation means be adapted forseparately receiving solar radiation.

In another embodiment it is preferred that the apparatus furthercomprises a means for separating solar radiation into a shorterwavelength component and a longer wavelength component, wherein:

(a) the electrical energy conversion means is adapted for receiving endfor converting the shorter wavelength component into electrical energy;and

(b) the thermal energy conversion means is adapted for receiving andconverting the longer wavelength component into thermal energy.

It is preferred that the solar radiation separating meats comprises amirror for selectively reflecting either the longer wavelength componentor the shorter wavelength component of the solar radiation spectrum.

It is preferred particularly that the mirror be positioned between thesolar radiation concentrating means and the electrical energy conversionmeans and that the mirror comprise a spectrally selective filter to makethe mirror transparent to the non-reflected component of the solarradiation spectrum.

It is preferred more particularly that the mirror be adapted forselectively reflecting the longer wavelength component of the solarradiation spectrum and that the spectrally selective filter be aninterference or edge filter to make the mirror transparent to theshorter wavelength component of the solar radiation spectrum.

It is preferred that the apparatus further comprises a non-imagingconcentrator for concentrating the reflected longer wavelength componentof the solar radiation spectrum.

It is preferred that the apparatus further comprises an optical fibre ora light guide for transferring the reflected longer wavelength componentof the solar radiation spectrum to the thermal conversion means.

It is preferred that the apparatus further comprises, a heat exchangemeans for extracting thermal energy from hydrogen, oxygen, and exhauststeam produced in the electrolysis cell and using the extracted thermalenergy as part of the energy component required for converting feedwater into steam or for pre-heating steam for consumption in theelectrolysis cell.

According to a second aspect of the present invention there is providedan apparatus for separating solar radiation into a longer wavelengthcomponent and a shorter wavelength component comprising, a mirror forselectively reflecting either the longer wavelength component or theshorter wavelength components of the solar radiation spectrum.

It is preferred that the mirror comprise, a spectrally selective filterto make the mirror transparent to the non-reflected component of thesolar radiation spectrum.

It is preferred that the mirror be appropriately curved so that it canconcentrate and direct the reflected longer wavelength component or theshorter wavelength component to a distant point for collection by areceiver.

It is preferred that the apparatus further comprises, a non-imagingconcentrator for concentrating the reflected longer or shorterwavelength component.

It is preferred that the apparatus further comprises, an optical fibreof light guide for transferring the concentrated reflected longer orshorter wavelength component for use in an end use application.

It is preferred particularly that the end use application be thegeneration of hydrogen by electrolysis of water.

The present invention is described further by way of example withreference to the accompanying drawings, in which:

FIG. 1 illustrates schematically one embodiment of an apparatus forproducing hydrogen in accordance with the present invention;

FIG. 2 illustrates schematically another embodiment of an apparatus forproducing hydrogen in accordance with the present invention;

FIG. 3 illustrates schematically a further embodiment of an apparatusfor producing hydrogen in accordance with the present invention;

FIG. 4 illustrates schematically a further embodiment of an apparatusfor producing hydrogen in accordance with the present invention;

FIG. 5 is a diagram which shows the major components of an experimentaltest rig based on the preferred embodiment of the apparatus shown inFIG. 1; and

FIG. 6 is a detailed view of the electrolysis cell of the experimentaltest rig shown in FIG. 4.

The basis of the first aspect of the present invention is to use solarenergy to provide the total energy requirements, in the form of athermal energy component and an electrical energy component, to formhydrogen and oxygen by the electrolysis of water. In this connection,the applicant has found that the combined effect of solar-generatedthermal energy and electrical energy results in a significantimprovement in the efficiency of the electrolysis of water in terms ofenergy utilisation, particularly when the thermal component is providedas a by-product of solar-generated electricity production.

The apparatus shown schematically in FIG. 1 is in accordance with thefirst aspect of the present invention and comprises, a suitable form ofsolar concentrator 3 which focuses a part of the incident solarradiation onto an array of solar cells 5 for generating electricity andthe remainder of the incident solar radiation onto a suitable form ofreceiver 7 for generating thermal energy.

The electricity and the thermal energy generated by the incident solarradiation are transferred to a suitable form of electrolysis cell 9 sothat:

(a) a part of the thermal energy converts an inlet stream of water forthe electrolysis cell 9 into steam and heats the steam to a temperatureof about 1000° C.; and

(b) the electrical energy and the remainder of the thermal energyoperate the electrolysis cell 9 to decompose the high temperature steaminto hydrogen and oxygen.

The hydrogen is transferred from the electrolysis cell 9 into a suitableform of storage tank 11.

The receiver 7 may be any suitable form of apparatus, such as a heatexchanger, which allows solar radiation to be converted into thermalenergy.

The apparatus shown in FIG. 1 further comprises a heat exchanger means(not shown) for extracting thermal energy from the hydrogen end oxygen(and any exhaust steam) produced in the electrolysis cell 9 andthereafter using the recovered thermal energy in the step of convertingthe inlet stream of water into steam for consumption in the electrolysiscell 9. It is noted that the recovered thermal energy is at a relativelylower temperature than the thermal energy generated by solar radiation.As a consequence, preferably, the recovered thermal energy is used topreheat the inlet water, and the solar radiation generated thermalenergy is used to provide the balance of the heat component required toconvert the feed water or steam to steam at 1000° C. and to contributeto the operation of the electrolysis cell 9.

It is noted that the component of the thermal energy which is usedendothermically at high temperature in the electrolysis cell 9 isConsumed at nearly 100% efficiency. This high thermal energy utilisationis a major factor in the high overall efficiency of the system. It isalso noted that high temperatures are required to achieve the highthermal energy efficiency and as a consequence only systems which cancollect and deliver thermal energy at high temperatures (700° C.+) canachieve the high efficiency.

The apparatus shown in FIG. 1 is an example of a parallel arrangement ofsolar cells 5 and thermal energy receiver 7 in accordance with the firstaspect of the present invention. The first aspect of the presentinvention is not restricted to such arrangements and extends to seriesarrangements of solar cells 5 and thermal energy receiver 7. Theapparatus shown schematically in FIGS. 2 to 4 are examples of suchseries arrangements. In addition, the apparatus shown schematically inFIGS. 2 to 4 incorporate examples of apparatus in accordance with thesecond aspect of the present invention.

The apparatus shown schematically in FIGS. 2 to 4 take advantage of thefact that solar cells selectively absorb shorter wavelengths and may betransparent to longer wavelengths of the solar radiation spectrum. Inthis connection, the threshold is in the order of 1.1 micron for siliconsolar cells and 0.89 micron for GaAs cells leaving 25% to 35% of theincoming energy of the solar radiation, which is normally wasted, foruse as thermal energy.

The apparatus shown in FIGS. 2 to 4, in terms of the first aspect of thepresent invention, in each case, is arranged so that, in use, solarradiation is reflected from a solar concentrator 3 onto a solar cell 15to generate electricity from the shorter wavelength component of thesolar radiation and the solar radiation that is not used for electricitygeneration, i.e. the longer wavelength component, is directed to athermal energy receiver (not shown) of an electrolysis cell 17 toconvert the solar radiation into thermal energy. The apparatus shown inFIGS. 2 to 4, in terms of the second aspect of the present invention, ineach case, comprises a means which, in use, separates the longer andshorter wavelength components of the solar radiation spectrum so thatthe components can be used separately for thermal energy and electricitygeneration, respectively.

The solar radiation separating means comprises a mirror 27 (not shown inFIG. 2 but shown in FIGS. 3 and 4) positioned in front of or behind thesolar cells 15.

In situations where the mirror 27 is positioned in front of the solarcells 15, as shown in FIGS. 3 and 4, the mirror 27 comprises aninterference filter or edge filter (not shown) which makes the mirror 27transparent to the shorter wavelength component of the solar radiationspectrum.

The mirror 27 may be of any suitable shape to reflect and selectivelydirect the longer wavelength component of the solar radiation spectrum.For example, in situations where the mirror 27 is positioned in front ofthe solar cells 15 and the focal point of the solar concentrator 3, asshown in. FIGS. 3 and 4, the mirror 27 may take the form of aCassigranian mirror, and in situations where the mirror 27 is positionedbehind the focal point of the solar concentrator 3, the mirror may takethe form of a Gregorian mirror.

The longer wavelength radiation reflected by the solar cells 15 may betransferred to the electrolysis cell 17 by any suitable transfer means21 such as a heat pipe (not shown) or an optical fibre (or light guide),as shown in FIGS. 2 and 4, or directly as radiation, as shown in FIG. 3.

With particular regard to the apparatus shown in FIG. 4, theelectrolysis cell 17 is positioned remote from the solar cells 15, andthe apparatus further comprises a non-imaging concentrator 33 forconcentrating the reflected longer wavelength component of the solarradiation prior to transferring the concentrated component to theoptical fibre or light guide 21.

It is also noted that the second aspect of the present invention is notlimited to use of the reflected longer wavelength component of the solarradiation spectrum to provide thermal energy to an electrolysis cell andmay be used to provide thermal energy in any end use application.

The electrolysis cells 9,17 shown in the figures may be of any suitableconfiguration. Typically, the electrolysis cells 9,17 are formed from amaterial, such as yttria stabilised zirconia (YSZ), which is porous tooxygen and impermeable to other gases, and the accessories, such asmembranes and electrodes (not shown), are formed from materials, such asalloys and cermets.

The apparatus of the present invention as described above take advantageof the facts that:

(a) the electrical potential and the electrical energy necessary toproduce hydrogen in an electrolysis cell decreases as the temperatureincreases and the balance of the energy requirements to operate theelectrolysis cell can be provided in the form of thermal energy;

(b) the efficiency of generation of thermal energy from solar radiationis significantly higher (in the order of 3 to 4 times) than theefficiency of generation of electricity from solar radiation; and

(c) the efficiency of consumption of the thermal energy endothermicallyin the electrolysis cell approaches 100%.

It is noted that it is believed by the applicant that the use of theby-product thermal energy can only be practically executed by the meansdescribed herein since other currently known methods are not capable oftransferring energy to produce a temperature in excess of 1000° C.

In other words, a particular advantage of the present invention is that,as a consequence of being able to separate the longer a shorterwavelength components of the solar radiation spectrum, it is possible torecover and convey and use that longer wavelength component in hightemperature application where otherwise that longer wavelength componentwould have been converted into low temperature heat (typically less than45° C.) and being unusable.

Further advantages of the present invention are as follows:

1. The efficiency of hydrogen production is greater than any other knownmethod of solar radiation generated hydrogen production.

2. The present invention increases the overall efficiency of the system,i.e. the efficiency of producing hydrogen by this method is greater thanthe efficiency of just producing electricity.

3. The present invention provides a medium, namely hydrogen, for theefficient storage of solar energy hitherto not available economicallyand thus overcomes the major technological restriction to large scaleuse of solar energy.

It should be noted that the performance of the present invention isexpected to exceed 50% efficiency. The theoretical performance is in theorder of 60%, whereas the existing technology is not expected topractically exceed 14% efficiency and has a threshold limit of 18%.

In order to illustrate the performance of the present invention theapplicant carried out experimental work, as described below, on anexperimental test rig shown in FIGS. 5 and 6 which is based on theembodiment of the apparatus shown in FIG. 1.

With reference to FIGS. 5 and 6, the experimental test rig comprised a1.5 m diameter paraboloidal solar concentrating dish 29 arranged totrack in two axes and capable of producing a solar radiation flux ofapproximately 1160 suns and a maxim temperature of approximately 2600°C. It is noted that less than the full capacity of power andconcentration of the concentrating dish 29 was necessary for theexperimental work and thus the receiving components (not shown) wereappropriately positioned in relation to the focal plane and/or shieldedto produce the desired temperatures and power densities.

The experimental rig further comprised, at the focal zone of the solarconcentrating dish 29, an assembly of an electrolysis cell 31, a tubularheat shield/distributor 45 enclosing the electrolysis cell 31, a solarcell 51, and a length of tubing 41 coiled around the heatshield/distributor 45 with one end extending into the electrolysis cell31 and the other end connected to a source of water.

The solar cell 51 comprised a GaAs photovoltaic (19.6 mm active area)concentrator cell for converting solar radiation deflected from theconcentrator dish 31 into electrical energy. The GaAs photovoltaic cellwas selected because of a high conversion efficiency (up to 29% atpresent) and a capacity to handle high flux density (1160 suns) atelevated temperatures (100° C.). In addition, the output voltage ofapproximately 1 to 1.1 volts at maximum power point made an ideal matchfor direct connection to the electrolysis cell 33 for operation at 1000°C.

With particular reference to FIG. 6, the electrolysis cell 31 was in theform of a 5.8 cm long by 0.68 cm diameter YSZ closed end tube 33 coatedinside and outside with platinum electrodes 35, 37 that formed cathodesand anodes, respectively, of the electrolysis cell 31 having an externalsurface area of 8.3 cm² and an internal surface area of 7.6 cm² .

The metal tube 45 was positioned around the electrolysis cell 31 toreduce, average and transfer the solar flux over the surface of theexterior surface of the electrolysis cell 31.

The experimental text rig further comprised, thermocouples 47 (FIG. 5)connected to the cathode 35 and the anode 37 to continually measure thetemperatures inside and outside, respectively, the electrolysis cell 31,a 1 mm² platinum wire 32 connecting the cathode 35 to the solar cell 51,a voltage drop resistor (0.01Ω) (not shown) in the circuit connectingthe cathode 35 and the solar cell 51 to measure the current in thecircuit, and a Yokogawa HR-1300 Data Logger (not shown).

The experimental test rig was operated with the electrolysis cell 31above 1000° C. for approximately two and a half hours with an excess ofsteam applied to the electrolysis cell 31. The output stream ofunreacted steam and the hydrogen generated in the electrolysis cell 31was bubbled through water and the hydrogen was collected And measured ina gas jar.

When a steady state was reached, readings of temperature, voltage,current and gas production were recorded and the results are summarisedin Table 1 below.

    ______________________________________           Electrolysis          Electrolysis           Cell      Electrolysis                                 Cell    Gas    Time   Voltage   Cell Current                                 Temperature                                         Production    ______________________________________           V         Amps        °C.                                         ml    2.22   1.03      .67         1020     0    2.39   1.03      .67         1020    80    net 17                               net 80 ml    minutes    ______________________________________

On the basis of the measured electrolysis cell voltage of 1.03 Vrecorded in Table 1 and a determined thermoneutral voltage of 1.47, theelectrical efficiency of the electrolysis cell 31, calculated as theratio of the thermoneutral and measured voltages, was ##EQU1##

In terms of the solar cell efficiency, with the solar cell 31 positionedto receive a concentration ratio of 230 suns and assuming:

(a) an output voltage=1.03 (=voltage across electrolysis cell and allowsfor connection losses);

(b) a current of 0.67 Amps;

(c) direct solar input is 800 w/m ^(;) 2 ; and

(d) an active solar cell area=19.6×10⁻⁶ m². the efficiency of the solarcell 51 (ηpv) was ##EQU2##

With a spectral reflectivity of 0.9 for the mirror surface of the solarconcentrating dish 29, the efficiency of the solar concentrator dish 29was 0.85.

Thus, the total system efficiency of the solar cell 51 and theelectrolysis cell 31 and optics (ηtotal) was

    ηtotal=0.85×0.19×1.43=0.22                 (22%)

The above figures of 22% is approximately twice the best previousproposed systems and more than three times the best recorded figure fora working plant.

The results of the experimental work on the experimental test rigestablish that:

(a) it is possible to produce hydrogen by high temperature electrolysisof water driven totally by solar radiation,

(b) the efficiency of production is greatly improved over known systems,and

(c) a significant portion of the heat of solar radiation can be useddirectly in the electrolysis reaction this reducing greatly expensiveelectrical input by almost half.

Many modifications may be made to the preferred embodiments of thepresent invention as described above without departing from the spiritand scope of the present invention.

By way of example, it is noted that, whilst the preferred embodimentsdescribe methods which convert water into hydrogen and oxygen, it canreadily be appreciated that the present invention is not so limited andextends to operating the methods in reverse to consume hydrogen andoxygen to produce thermal energy and electricity. In this regard, it hasbeen found by the applicant that under certain conditions the electricalinput required to produce a unit of hydrogen in accordance with thepreferred embodiments of the method is less than the electrical outputproduced when the hydrogen is used in the methods arranged to operate inreverse and thus as well as the system producing hydrogen the overallelectrical efficiency of the plant can also be enhanced.

Furthermore, whilst the preferred embodiments describe the use of solarcells to convert solar energy into electricity, it can readily beappreciated that the present invention is not so limited and extends toany suitable solar radiation to electricity converters.

Furthermore, whilst the preferred embodiments describe that the secondaspect of the present invention separates the longer and shorterwavelength components of the solar radiation spectrum by reflecting thelonger wavelength component, it can readily be appreciated that thesecond aspect of the present invention is not limited to such anarrangement and extends to arrangements in which the shorter wavelengthcomponent is reflected.

I claim:
 1. A method of producing hydrogen by the electrolysis of steam,the method comprising, converting solar radiation into thermal energyand electrical energy, and using a part of the thermal energy to convertwater into steam and to heat the steam to a temperature of at least 700°C., and using the electrical energy and the remaining part of thethermal energy to operate an electrolysis cell to decompose the steamand to produce hydrogen and oxygen, with the thermal energy providing atleast a part of the endothermic component of the electrolysis reactionand to significantly reduce the additional external electrical energyrequired to operate the electrolytic cell and increasing the efficiencyof hydrogen production.
 2. The method defined in claim 1 comprising,separating the solar radiation into a shorter wavelength component and alonger wavelength component, and converting the shorter wavelengthcomponent into electrical energy and converting the longer wavelengthcomponent into thermal energy.
 3. The method defined in claim 1 or claim2 further comprising, using solar radiation generated thermal energy forconverting water into steam and for operating the electrolysis cell andusing solar radiation generated electrical energy for operating theelectrolysis cell.
 4. The method defined in claim 3 further comprising,extracting thermal energy from hydrogen, oxygen, and exhaust steamproduced in the electrolysis cell and using the extracted thermal energyas part of the energy component required for converting water into steamand for pre-heating steam for consumption in the electrolysis cell. 5.An apparatus for producing hydrogen by electrolysis comprising:anelectrolysis cell having an inlet for steam and outlets for hydrogen,oxygen, and excess steam, means for separating solar radiation, bywavelength, into a first wavelength component and a second wavelengthcomponent, said first wavelength component being relatively shorter thansaid second wavelength component, electrical energy conversion means forreceiving and converting said first (shorter) wavelength component intoelectrical energy, and thermal energy conversion means for receiving andconverting said second (relatively longer) wavelength component intothermal energy, said electrical energy conversion means and said thermalenergy conversion means being arranged in series or in parallelrelationship for providing energy required for converting water intosteam and for operating the electrolysis cell to decompose the steaminto hydrogen and oxygen at high temperature of at least 700°C.
 6. Theapparatus defined in claim 5 wherein, the electrolysis cell is at leastpartially formed from materials that allow oxygen to be separated fromhydrogen in or adjacent to the electrolysis cell.
 7. The apparatusdefined in claim 5 or claim 6, further comprising, a means forconcentrating solar radiation on the thermal energy conversion means andon the electrical energy conversion means.
 8. The apparatus defined inclaim 7 wherein, the electrical energy conversion means and the thermalenergy conversion means are adapted for separately receiving solarradiation.
 9. The apparatus defined in claim 5, wherein the solarradiation separating means comprises, a mirror for selectivelyreflecting either said second (relatively longer) wavelength componentor said first (shorter) wavelength component of the solar radiationspectrum.
 10. The apparatus defined in claim 9, wherein the mirror ispositioned between the solar radiation concentrating means and theelectrical energy conversion means and the mirror comprises a spectrallyselective filter to make the mirror transparent to the non-reflectedcomponent of the solar radiation spectrum.
 11. The apparatus defined inclaim 9, wherein the mirror is adapted for selectively reflecting saidsecond (relatively longer) wavelength component of the solar radiationspectrum.
 12. The apparatus defined in claim 11, wherein the spectrallyselective filter comprises an interference or edge filter.
 13. Theapparatus defined in claim 9 further comprising, a non-imagingconcentrator for receiving and further concentrating the reflectedcomponent of the solar radiation spectrum.
 14. The apparatus defined inany one of claim 9 to 13 further comprising, a means for conveying thereflected component of the solar radiation spectrum to the thermalenergy conversion means.
 15. The apparatus defined in claim 14, whereinthe conveying means comprises an optical fibre or a light guide.
 16. Theapparatus defined in claim 5 wherein the electrical energy conversionmeans is a solar cell.
 17. The apparatus defined in claim 5 furthercomprising, a heat exchange means for extracting thermal energy fromhydrogen, oxygen, and exhaust steam produced in the electrolysis celland using the extracted thermal energy to form steam for consumption inthe electrolysis cell.
 18. The apparatus defined in claim 5, wherein theapparatus is reversible so that hydrogen and oxygen can be reactedtogether to produce heat and electricity.